ISO FAR-IR Spectroscopy of IR-Bright Galaxies and Ulirgs
NAVAL RESEARCH LAB WASHINGTON DC
Pagination or Media Count:
Based on far-infrared spectroscopy of a small sample of nearby infraredbright and ultraluminous infrared galaxies ULIRGs with the ISO Long Wavelength Spectrometer 1 we find a dramatic progression in ionicatomic fine-structure emission line and molecularatomic absorption line characteristics in these galaxies extending from strong O III52,88 um and N III57 um line emission to detection of only faint C II158 um line emission from gas in photodissociation regions in the ULIRGs. The molecular absorption spectra show varying excitation as well, extending from galaxies in which the molecular population mainly occupies the ground state to galaxies in which there is significant population in higher levels. In the case of the prototypical ULIRG, the merger galaxy Arp 220, the spectrum is dominated by absorption lines of OH, H2O, CH, and O I. Low O III88 um line flux relative to the integrated far-infrared flux correlates with low excitation and does not appear to be due to far-infrared extinction or to density effects. A progression toward soft radiation fields or very dusty H II regions may explain these effects.